The present invention is concerned with the improvement in the curing of epoxy resin systems which have heretofore been catalyzed by tertiary amines, and more specifically to new epoxy resin systems which contain a half-ester compound as a latent catalyst.
Aliphatic tertiary amines, quaternary ammonium salts and organic metal salts have conventionally been used as curing catalysts for epoxy resins. However, as is well known in the art, these curing catalysts shorten the shelf life of the epoxy resin systems. Accordingly, so-called latent curing catalysts that are very stable at temperatures approximating room temperature without causing the shortening of the shelf life but rapidly cure the epoxy resins under heating have been proposed and used advantageously in the art. As typical instances of such latent curing catalysts, there can be mentioned products formed by converting tertiary amines to salts by boron trifluoride, a carboxylic acid or an onium compound, and compounds containing a basic nitrogen atom and a carboxyl group. In fact, these catalysts are considerably improved in the latent curing property. A Japanese Laid-open Publication No. 10399 of 1975 (claiming priority of U.S. application S.N. 352201, filed Apr. 18, 1973) discloses salts of polycarboxylic acid anhydride with imidazole compounds. The salts are used for epoxy resins as a curing agent.
A U.S. Pat. 3,497,524 also discloses various salts obtained from tertiary amines and halides of phthalic acid. In these days, a higher latent curing property is demanded. Cured epoxy resins obtained by using conventional curing catalysts have been found defective because an undesirably large amount of a leak current is generated when the epoxy resin composition is applied to semiconductor devices.